Police used anti-terrorism laws to arrest five people they suspect of planning to disrupt Thursday's G20 summit, it emerged yesterday.

Three men and two women, who were thought to be political activists, were taken into police custody on Friday after police officers discovered imitation weapons, fireworks and political literature during a search of a flat in Plymouth.

The investigation was sparked after a 25-year-old man was arrested for allegedly spraying graffiti on a wall in Plymouth city centre, prompting officers to carry out a search of his flat. There they discovered a number of suspicious items including a replica Kalashnikov assault rifle and several minor explosive devices made from fireworks.

A 16-year-old schoolboy and two women aged 20, who were in the flat at the time, were also detained under the Terrorism Act and for separate drugs offences. Further investigations led to the arrest of a 19-year-old man on Sunday.

The arrests are the first direct police action ahead of Thursday's G20 summit, which will see the leaders of the world's 20 richest countries, including US President Barack Obama, converge on London. The Metropolitan Police has cancelled all officers' leave this week, in anticipation of the capital's most comprehensive security operation in a decade.

Up to 3,000 officers will be posted across the capital during the summit, for which the total policing bill will be more than £7.2m. The Metropolitan Police Commissioner, Sir Paul Stephenson, has warned that an "unprecedented" level of protest is likely to stretch his force to the limit.

Yesterday the security operation moved up a gear, with workers seen boarding up shop-fronts in the City at the request of proprietors worried about damage being done to their premises by protesters. Meanwhile, it also emerged that those who live near the G20 venue in Canning Town have been advised to carry photo IDs with them to ensure they can pass through police roadblocks and access their homes.

Commenting on yesterday's arrests, a spokesman for the Met said: "At the current time we have no information to suggest a change to the threat picture facing either the demonstrations or G20."

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